(a) Field of the Invention
The present invention relates to an objective for video disks comprising three lens elements, which is small and light in the weight and has a large N.A.
(b) Description of the Prior Art
In general, objectives for video disks for performing automatic collimation are desired to be small and light in the weight since they are used in movable portions. Further, in order to make a beam spot small, it is favourable to have a large N.A. Particularly, in case a semi-conductor laser as a light source, due to the long wavelength of light beam from the light source, a small beam spot will not be obtained if N.A. is not large.
A known conventional objectives for video disks comprising three lens elements, there are such lenses as disclosed in Japanese Patent Laid-Open Nos. 71253/77, 97750/77 and 67445/79. These objectives have N.A. 0.5, 0.4 and 0.4 respectively and, therefore, they cannot be said yet as having enough large N.A. As a conventional objective for video disks having large N.A., there is such lens as disclosed in Japanese Patent Laid-Open No. 31754/77. However, as this objective comprises five lens elements, it is heavy and high in the manufacturing cost.